1. Field of the Invention
The subject invention is directed to force motors as used in fluid power systems and, more particularly, force motors wherein electro-magnetic coils are used to bias the field strength of a permanent magnet.
2. Description of the Prior Art
Controls for hydraulic power systems have had a long history of development. Early control systems were primarily mechanical linkages. These systems were reliable, but tended to be heavy, bulky and somewhat limited in capabilities. Also, as mechanical control systems grew in size and complexity they became increasingly costly to manufacture and maintain.
As an alternative to mechanical systems, electrical control systems became increasingly popular, particularly in aviation and related fields. Electrical control systems generally could be made smaller, lighter and more versatile than mechanical systems. However, electrical control systems had other disadvantages. For example, the quiescent leakage of electro-hydraulic valves was relatively high. Consequently, such systems required more power, generated more heat, and were generally more costly. Applications requiring redundancy in the control system, such as aviation applications, merely compounded these factors with a multiplicity of components in complex redundancy management systems.
Accordingly, it was recognized in the prior art that a mechanism that directly controlled hydraulic valves would be more efficient in terms of quiescent leakage and thus have many advantages over hydraulic control systems known in the prior art. Moreover, such use of direct drive valves would increase reliability and decrease bulk and weight of the hydraulic system. In addition, it was also recognized that direct drive valves would require only limited failure monitoring for the control system, resulting in a correlative improvement in redundancy management.
Early direct drive valves employed force motors in which a magnetic assembly comprised of electrical coils was used to control the position of an armature. Subsequently the electrical coils were replaced by a permanent magnet in combination with several smaller electrical coils that were used to bias the field of the permanent magnet. It was found that this provided a magnetic assembly that was lighter and had lower power requirements than prior magnetic assemblies having no permanent magnets.
Previously, direct drive valves were developed having much-improved quiescent leakage characteristics typically in the range of 10% to 1%. One example is shown in a paper entitled "Application and Use of Rare Earth Magnets," by M. F. Marx, prepared for SAE Aerospace Control and Guidance Systems Committee, meeting No. 41, Palo Alto, Calif. However, several disadvantages remained in force motors known to the prior art. For example, some force motors had no mechanism for isolating the electrical coils of the magnetic assembly from the fluid of the hydraulic system. This exposure to hydraulic fluid made the magnetic assembly subject to premature failure. Other persistant problems with force motors have included a requirement for relatively high threshold command signals to initiate movement of the armature from a stationary position, as well as hysteresis in the armature movement relative to control current. These problems adversely effected the performance characteristics of the force motor, particularly sensitivity and stability.
Accordingly, there was a need in the prior art for a force motor that was suitable for use in a direct drive valve and that would overcome the threshold, hysteresis and other disadvantages of force motors known in the prior art.